Identifying and tailoring C–N coupling site for efficient urea synthesis over diatomic Fe–Ni catalyst

Electrocatalytic urea synthesis emerged as the promising alternative of Haber–Bosch process and industrial urea synthetic protocol. Here, we report that a diatomic catalyst with bonded Fe–Ni pairs can significantly improve the efficiency of electrochemical urea synthesis. Compared with isolated diatomic and single-atom catalysts, the bonded Fe–Ni pairs act as the efficient sites for coordinated adsorption and activation of multiple reactants, enhancing the crucial C–N coupling thermodynamically and kinetically. The performance for urea synthesis up to an order of magnitude higher than those of single-atom and isolated diatomic electrocatalysts, a high urea yield rate of 20.2 mmol h−1 g−1 with corresponding Faradaic efficiency of 17.8% has been successfully achieved. A total Faradaic efficiency of about 100% for the formation of value-added urea, CO, and NH3 was realized. This work presents an insight into synergistic catalysis towards sustainable urea synthesis via identifying and tailoring the atomic site configurations.


They have concluded "
The current work demonstrated for the first time the efficient urea synthesis on a bonded Fe-Ni pairs enriched diatomic electrocatalyst with a high urea yield rate of 20.2 mmol h -1 g-1 and FE of 17.8%. The competing hydrogen evolution was in-situ suppressed with a total FE of about 100% for the value-added product formation of CO,NH3 and urea." In the second sentence they have indicated total 100% FE for CO, NH3 and urea formation over HER. What is the total Faradaic efficiency? Please clarify.
2. They have indicated that over the bonded Fe-Ni-DASC the FE of CO, NH3 and urea formation are 73.3%, 77.6% and 17.8% respectively at -1.5V. Will CO and NH3 formation not compete with urea formation? If the reactions compete how can this study provide the efficient route for urea synthesis suppressing those reactions?
3. It is observed that urea synthesis is performed in presence of nitrate or nitrite and CO2. But in SR-FTIR measurements there is a sign of moderate NO adsorption on the B-FeNi-DASC. They have also theoretically studied urea synthesis from NO and CO2 over the same catalytic surface. What is the source of this adsorbed NO? If it is obtained from NO3-or NO2-please include the relative free energy diagram of NO formation. 4. Please indicate the theoretical adsorption energy and/or free energy of adsorption of CO2, NO, nitrate, nitrite and CO over the SACs and isolated/bonded-Fe-Ni-DASCs. 5. Please mention the relative free energies of all the adsorbed species and the intermediates in the free energy diagram in . Please include the clear side and above views of the electronic structures of the adsorbed species and intermediates over the catalytic surface, so that a clear overview of the structures can be perceived. Figure-23 authors have presented the differential charge density maps of the isolated/bonded-DASCs. Please indicate the iso-value of these maps.

In Supplementary
8. In this context the following articles can be consulted for discussion, a. https://dx.doi.org/10.1021/acs.jpclett.1c00421 b. https://dx.doi.org/10.1021/acs.jpclett.1c03242 c. https://dx.doi.org/10/1021/acs.jpcc.1c08779 Reviewer #2 (Remarks to the Author): In this manuscript, the authors demonstrated an important strategy of oxygen vacancies towards electrocatalytic urea synthesis, and clarified the reaction mechanism. It was found that oxygen vacancies inhibited the hydrogenation reaction by anchoring the *NO intermediates, thereby promoting the C-N coupling process. The reaction mechanism was supported by in situ electrochemical characterization via tracing the reaction intermediate species. Since the precise quantification of urea is an effective prerequisite for the development of this field, the authors have investigated the potential impact of multiple by-products on the urea quantification, and demonstrated the most reliable urea quantification protocol, contributing to the development of related fields. Overall, this work is interesting with insightful information. The reviewer would like to recommend its acceptance after addressing the following technical points.
1. The authors demonstrated that the urea decomposition method is the most suitable one to quantify the electrocatalytic generated urea. Under certain urea concentration, whether the ammonia obtained by urease decomposition is reliable or not? The author needs to provide the source of urease, including the manufacturer and production batch, so that other researchers could reproduce the results more easily.
2. The urease decomposition method was adopted in this work for the quantification of urea product. The UV-vis adsorption curves for the urea quantification need to be complemented in the Supplementary Information.
3. The structural stability is highly important for the recognition of active sites and the evaluation of catalyst stability. The author needs to confirm whether there is metal leaching under the working condition.  Except edge position drifting, other XANES features have been experimentally revealed by the reported XANES data, which is yet not fully addressed. Fe and Ni K edges XANES reported by SI Fig.6 (a) & (b) reveal well resolved XANES difference throughout the sample system in (1) pre-edge feature, (2) lineshpae throughout the edge jump, (3) peak drifting, peak splitting, overall lineshape changing, and peak amplitude varying of XANES whiteline, and (4) significant difference in the post whiteline XANES region. These well resolved features are specific to the Fe and Ni local coordination environments, featured by their occupied sites, respectively. It is suggested to further address these experimentally revealed XANES features, correlating them to the local structural environment for the Fe and Ni occupied site predicted by DFT.
Some other further comments:

Reviewer #1 (Remarks to the Author):
Through this article entitled "Identifying and tailoring C-N coupling sites for efficient urea synthesis on diatomic Fe-Ni catalyst" the authors have attempted to provide a solution to a very challenging problem of direct synthesis of urea through coupling of Carbon and Nitrogen (bypassing the harsh reaction conditions of the Haber-Bosch process of synthesis of NH3). They have synthesized the catalytic systems and studied the urea synthesis paths.
Their work is also validated by DFT calculations. But there are some ambiguities and addressing those in the revised manuscript will surely increase the impact of this manuscript.
Thanks for your approval to this paper and we have made corresponding revisions according to your instructive comments, the details are afforded as followed. and urea formation over HER. What is the total Faradaic efficiency? Please clarify.

Response:
We are really appreciating for your instructive comments. The above expressions regarding "total Faradaic efficiency" may cause confusion for the readers. The total Faradaic efficiency in this work refers to the efficiency for the formation of value-added CO, NH3 and urea products.
In the revised manuscript, the sentence of "The competing hydrogen evolution is in-situ suppressed by C-N coupling with a total Faradaic efficiency of about 100% for the valueadded products." has been revised to "A total Faradaic efficiency of about 100% for the formation of value-added urea, CO and NH3 was realized." The sentence of "Meanwhile, the total FE of the formation of the urea, CO and NH3 reaches about 100%, implying that the competing HER over B-FeNi-DASC system is successfully in-situ suppressed during the coupling reaction." has been corrected to "Meanwhile, the total FE for the formation of value-added CO, NH3 and urea products reaches about 100%, negligible hydrogen was produced in the co-electrolysis measurement over B-FeNi-DASC system, implying that the competing HER is almost completely suppressed." 2. They have indicated that over the bonded Fe-Ni-DASC the FE of CO, NH3 and urea formation are 73.3%, 77.6% and 17.8% respectively at -1.5 V. Will CO and NH3 formation not compete with urea formation? If the reactions compete how this study can provide the efficient route for urea synthesis suppressing those reactions?

Response:
Thanks for your comments. The electrocatalytic urea synthesis involves the co-activation of carbon dioxide and nitrate. High carbon dioxide or nitrate reduction activity alone leads to selective activation of reactive species on the catalyst surface, as a result, the reaction rate and the concentration of active intermediate species (carbon-containing and nitrogencontaining) involved in the coupling reaction do not match, resulting in the low urea synthesis activity for the single-atom electrocatalysts. Therefore, we designed a diatomic catalyst (FeNi-N6) with good catalytic activity for both carbon dioxide reduction and nitrate reduction. During co-electrolysis measurements, the considerable amount of *CO and *NH species can be formed on the surface. More importantly, the thermodynamic spontaneous reaction between *CO and *NH conducive to the C-N coupling and subsequent urea synthesis.
The formation of CO and NH3 provides direct evidence for the generation of carbon- 3. It is observed that urea synthesis is performed in presence of nitrate or nitrite and CO2.
But in SR-FTIR measurements there is a sign of moderate NO adsorption on the B-FeNi-DASC. They have also theoretically studied urea synthesis from NO and CO2 over the same catalytic surface. What is the source of this adsorbed NO? If it is obtained from NO3or NO2please include the relative free energy diagram of NO formation.

Response:
We thank the reviewer for raising such a meaningful question. The source of this adsorbed NO is nitrate reduction reaction. In our revised manuscript, we calculated the free energy diagram of NO formation with NO3as a starting point. [R2,R3] Infer from the Supplementary The reaction path of NO3reduction to *NO follows the above references.

Response:
We thank the reviewer for pointing this out. In our revised manuscript, the following words

Response:
Thank you for reminding us the discipline of data presentation. The free energy diagram of nitrate reduction to *NO has been supplemented. The relative free energies of all intermediates involved along the urea formation are clearly checked and added to the revised Figure 4a.

Response:
Thank you for raising this important issue. We supplied the iso-value of the differential charge density maps in our revised manuscript.
The words "The charge density difference with iso surface value of 0.01 e/Bohr 3 of FeNi-N6 and FeN4-NiN4 respectively. Blue and yellow colors represent losing and gaining electrons respectively." have been supplemented in Supplementary Figure 30.

Reviewer #2 (Remarks to the Author):
In this manuscript, the authors demonstrated an important strategy of oxygen vacancies towards electrocatalytic urea synthesis, and clarified the reaction mechanism. It was found that oxygen vacancies inhibited the hydrogenation reaction by anchoring the *NO intermediates, thereby promoting the C-N coupling process. The reaction mechanism was supported by in situ electrochemical characterization via tracing the reaction intermediate species.
Since the precise quantification of urea is an effective prerequisite for the development of this field, the authors have investigated the potential impact of multiple byproducts on the urea quantification, and demonstrated the most reliable urea quantification protocol, contributing to the development of related fields. Overall, this work is interesting with insightful information. The reviewer would like to recommend its acceptance after addressing the following technical points.
Much appreciate for your approval to our work. We have supplemented the data and explanations to address the mentioned technical points.
1. The authors demonstrated that the urea decomposition method is the most suitable one to quantify the electrocatalytic generated urea. Under certain urea concentration, whether the ammonia obtained by urease decomposition is reliable or not? The author needs to provide the source of urease, including the manufacturer and production batch, so that other researchers could reproduce the results more easily.

Response:
Thanks for your comments. The urease activation and dosage are applied for fully decomposition of urea into ammonia, and the subsequent ammonia quantification is not be disturbed in this work. In order to verify the reliability of urease decomposition method used in this work. We prepared a series of urea standard solutions (concentration of 5 ppm, close to experimental value), these urea standard solutions (10 batches) were decomposed into ammonia by urease with the same conditions. The concentration of ammonia obtained by the indophenol blue method is listed in Figure R1b. It was found that the ammonia concentrations from 10 batches remain unchanged, indicating that the ureaderived ammonia quantification method with urease decomposition is stable. The urea quantification results by urease decomposition-indophenol blue method are matched with that of results by urease decomposition-NMR method, as illustrated in Supplementary   Figure 17, implying the applicability of urease decomposition for urea quantification. More important, the recent report also demonstrated that the urease decomposition-indophenol method is the reliable one for quantifying urea without interference from coexisting byproducts (J. Am. Chem. Soc. 2022, 144, 11530-11535).
The manufacturer and production batch of urease were list and added in revised manuscript. The following words "Chemicals. Carbon papers (HCP020N  3. The details for the quantification of nitrite ions in the electrolytes should be supplemented.

Response:
Thanks for your comments. A series of NO2with various concentrations were prepared and corresponding calibration curves were obtained in Supplementary Figure 34.
The nitrite quantification details of "NO2was determined using N-(1-naphthyl)ethylenediamine dihydrochloride spectrophotometric method with some modification. 4. The authors have evaluated the accuracy of the quantitative methods of the products, and concluded that the diacetyl monoxime method was susceptible to interference of nitrite ions and the urease decomposition method was the reliable one. However, the influence of carbon-containing products on the detection method was not considered, for example, common formic acid, methanol, and ethanol. Further evaluation of the impact of the above possible products on quantitative methods is needed to expand the scope of application of the conclusion.

Response:
Thanks for your comments. To verify whether the carbon-containing products (formic acid, methanol and ethanol) will affect the urease decomposition detection method, we prepared a series of urea standard solutions (concentration of 5 ppm) with addition of 50 mM formic acid, 50 mM methanol and 50 mM ethanol respectively. Then, these urea standard solutions with added carbon-containing products were decomposed by urease method, the obtained corresponding absorbance curve by UV spectrophotometer of these solutions with and without urease decomposition were compared and list in Figure R2. It is found that these UV absorbance curves before and after addition of carbon-containing products are basically coincident, indicating that the influence of carbon-containing products on the urease decomposition detection method is almost negligible. 5. In the Supporting Information, "In site sum frequency generation" should be "In situ sum frequency generation". In the Figure S5d and the Figure S6, the units for the abscissa are missing, which needed to be supplemented. The authors should also check the whole manuscript to correct some of typos.

Response:
Thanks for your comments. According to your suggestions, the whole manuscript has been carefully checked, the grammar and spelling mistakes have been thoroughly corrected.

Reviewer #3 (Remarks to the Author):
General comments for XANES: Except edge position drifting, other XANES features have been experimentally revealed by the reported XANES data, which is yet not fully addressed.
Fe and Ni K edges XANES reported by SI Fig.6 (a) & (b) reveal well resolved XANES difference throughout the sample system in (1) pre-edge feature, (2) lineshpae throughout the edge jump, (3) peak drifting, peak splitting, overall lineshape changing, and peak amplitude varying of XANES whiteline, and (4) significant difference in the post whiteline XANES region. These well resolved features are specific to the Fe and Ni local coordination environments, featured by their occupied sites, respectively. It is suggested to further address these experimentally revealed XANES features, correlating them to the local structural environment for the Fe and Ni occupied site predicted by DFT.

Response:
Thanks for your precious comments. XANES is kind of powerful tool to analysis materials' structure and reveal the coordination environments of active sites. We're sorry that we didn't process the XANES data professionally and analysis it well due to our lack of XANES knowledge. In the revised version, according to your suggestions, we have re-fit and further analyzed the XANES data to obtain more detailed structure information on the Fe and Ni local coordination environments to correlate them with DFT results well. Meanwhile, we will strengthen the study and understanding of XANES knowledge to better reveal the intrinsic active sites and catalytic mechanism of electrocatalysis resort to the powerful means.
Some other further comments: 1. Related contents: "peak position situates at lower energy than those of Fe foil…"in Manuscript, Page 3. Comments: A typo: "lower" should be replaced by "higher".

Response:
We are sorry for the mistake, and the "lower" have been revised to "higher" in the revised version. The following sentence of "the X-ray absorption near edge structure (XANES) data at the Fe K-edge of Fe-SAC, I-FeNi-DASC and B-FeNi-DASC clearly indicate that the peak position situates at lower energy than those of Fe foil ( Supplementary Fig. 6)" have been revised to "the X-ray absorption near edge structure (XANES) data at the Fe K-edge of Fe-SAC, I-FeNi-DASC and B-FeNi-DASC clearly indicate that the peak position situates at higher energy than those of Fe foil ( Supplementary Fig. 6)" in the revised manuscript.
2. Related contents: X-ray absorption fine structure (FT-EXAFS) analysis Comments: There is no analysis here, simply presenting magnitude of FT.

Response:
Thanks for your comments, we further analyzed the X-ray absorption fine structure (FT-EXAFS) and the following words "In terms of extended X-ray absorption fine structure (EXAFS) of the catalyst of B-FeNi-DASC, the predominant peaks appeared in the first coordinated shells (1-2 Å) upon R space curve of Fe K-edge and Ni K-edge, which originate from the scattering of 1st shell Fe-N and Ni-N path, are almost the same in position (at ~1.5 Å) and magnitude, indicating nearly identical coordination environment for Fe and Ni atoms in the catalyst of B-FeNi-DASC. Notably, the broad peak appears upon the 2nd shell of Fe K-edge and Ni K-edge with a single-scattering path of at around 2.49±0.07Å appeared in the second scattering shells (2-3.5 Å) (Fig. 1d, Supplementary Fig. 7 and Supplementary Table 1). This distance is in the range of the observed separation of dualatom pairs in atomic resolution STEM imaging (Fig. 1c), which is consistent with the previous reports, [25,26]

Response:
According to your suggestion, the arrow has been added as present in Figure R3, and the corresponding figure has been revised in the manuscript.

Response:
Thank for your comments. Indeed, it is unreasonable on the expression of "site configuration" without three-dimensional structural information as support, therefore, we change the "site configuration" to "Fe-Ni pairs". And the expression was also revised in XANES discussion section in the manuscript.
5. Related contents: Supplementary Figure 7 (SI, page 8) Comments: SI Fig. 7(a) vs (b): Better quality in EXAFS R space curve fitting was obtained upon the 2nd shell between ~R2-3 Å for Ni K edge data vs that for Fe K edge regarding sample B-FeNi-DASC. Please address whether the difference in fitting quality between two fitting is induced by the defect of the DFT model, which was used to guide the reported fitting, either structurally and/or chemically, or induced by other reason(s)?

Response:
Thanks for your comments. After comprehensive analysis, we deduced that the main reason caused the difference in fitting quality upon the 2nd shell between ~R2-3 Å for Ni K edge and that for Fe K edge is induced by unsuitable parameter setting carried on the

Response:
Thanks for your comments. The error bars of EXAFS curve-fitting parameters have been added as follow.
The electrocatalytic C-N coupling serves as a promising alternative to the traditional urea synthesis and the emerging field of electrocatalysis research. In this work, the authors designed a diatomic catalyst with bonded Fe-Ni pairs to improve the efficiency of electrochemical urea synthesis. The "three-in-one" of active site, activation site and coupling site was realized in the FeNi-N6 sites. The origin of the superior electrocatalytic activity for FeNi-N6 was experimentally and theoretically confirmed. This innovative research work achieved the identification and tailoring of C-N coupling sites for electrocatalytic urea synthesis, which is expected to promote the green revolution of urea industrial and provide broaden guidance for catalytic coupling reactions. Considering the significance of this work in the related fields, I would like to suggest its publication in Nature Communications after a minor revision.
Thanks for your approval to this manuscript. The information of the materials has been supplemented and the negligible leaching of metals during electrochemical measurements has been verified.
1. The information of the materials used must be afforded in the section of methods, including the purity of carbon dioxide gas, the manufacturer and purity of chemical reagents.

Response:
Thanks for your comments. The following words "Chemicals. Co., Ltd. All chemicals were used without further purification." have been added in the section of Methods in the revised manuscript.
2. The urease decomposition method was adopted in this work for the quantification of urea product. The UV-vis adsorption curves for the urea quantification need to be complemented in the Supplementary Information.

Response:
Thanks for your comments. The UV-vis adsorption curves for the urea quantification are 3. The structural stability is highly important for the recognition of active sites and the evaluation of catalyst stability. The author needs to confirm whether there is metal leaching under the working condition.

Response:
Thanks for your comments. In order to confirm whether there is metal dissolution during the process of electrochemical synthesis of urea, the electrolyte after 5 h continuous test was extracted and the metal content was measured by Inductive Coupled Plasma Emission Spectrometer (ICP). The test protocol is presented as followed: 5 mL electrolyte mixed with 5 mL HNO3 solution (8 M), and then the mixed solution was transferred to hydrothermal reactor under 180 °C for 24 h. After filtered by filter paper with a diameter of 0.22 μm, the obtained filtrate was conducted to detect the ion concentration of Fe and Ni leaching. The concentration of metals is lower than the detection line due to the ultra-low content. The results indicate that there is barely leaching of metal ions into the electrolyte after long-term measurement, implying the superior electrochemical stability of B-FeNi-DASC.
4. The quality of Figures needs to be further improved, such as Figure 1c and Figure 1d.

Response:
Thanks for your comments. The quality of figures has been improved and the revised

Response:
Thanks for your comments. We have corrected Figure 2b and revised in manuscript. And we have carefully checked the expressions in the whole manuscript. We would like to express our sincere thanks to the reviewers and editorial team of Nature Communications, and we think the quality of this manuscript has been significantly promoted attributing to the instructive comments.